So I wonder--is his behavior just a manifestation of his mental illness, and the Josh I once knew is still there somewhere, or is this who he really is?
I have always believed in nurture too. I think it would be hard to adopt if you did not believe strongly that your nurturing could make your child your own and make a difference for them.
Many years ago I dated one of a set of identical twins. And then, when that relationship was over I dated the other twin, unbeknownst to the first. I got to know each of them well. They were NOTHING alike. I am not saying that genetic influences were not present, but they were just that, influences. The major aspects of their personality seemed determined by choices and decisions and interests and motivations.
I agree with the others that the bipolar illness is the major influencer in Josh right now and until he gets some serious help with that he will continue off of the rails.
I think what needs to happen now is EXACTLY where you seem to be headed. To stop the ability of Josh to affect you. I would STOP any interaction with him, as he is being beyond abusive. I would have NO expectations of him whatsoever. You set yourself up this way. He is not doing that. You are.
He is not responsible really for what he is doing, if it is being directly or fueled by manic episodes. But that is not the same thing as saying he should not be held responsible. You must.
Right now this is between him and your sister who was very kind to take him in. We have had this happen too. My son for two years was harbored by friends (ex) who had a beachfront motel in a major tourist city. This did not end well. While the people were well-meaning (on the face of it) they only enabled my son. And set him up for a fall.
As I see this the likely outcome for people run amok with bipolar is that they come to a careening stop when they engage with authorities one way or another, either hospitalized involuntarily or arrested. Of course, there is the possibility that Josh manages to skirt this, and somehow his behavior moderates. But based upon my own experience, well-doers cannot help our children who are dealing with the kinds of issues that they deal with. What helps them is containment and treatment.
You have no control here as I see it. Nor do you have a role. Josh is not allowing it. Because his mental state is trumping all other things.
Your job is to not write the end of the story. As is my own. I have a really hard time not feeling that my own life is a tragedy as long as I feel that my son is beyond my help and in real trouble. My job too is to create safety for myself first and foremost, and then to create psychic safety by not battering myself and tarring myself with the brush of my son's suffering. *Sorry for mixed metaphors. I see you dealing with the same thing. This is an issue in us. Not in them.